1. Field of Invention
This invention relates to an ion implantation method and, in particular, the method can improve the uniformity of ion implantation.
2. Background of the Related Art
The different stages of an implantation method according to the prior art are shown in FIG. 1a˜FIG. 1d. An ion beam scans a substrate S along a scan path to implant ions onto the substrate surface and forms a plurality of parallel ion implantation scan lines on the surface of the substrate S, that is called an ion implantation scan pass.
As shown in FIG. 1a for explaining the execution of one ion implantation scan pass, the ion beam implants ions on the substrate S along a first direction, the direction parallel to that of a first scan path 21, to form an ion implantation scan line. And then the ion beam shifts with a scan pitch T in a second direction perpendicular to the first direction. Continuously, the ion beam implants ions onto the substrate S along the reverse direction of the first direction to form another ion implantation scan line. The procedure is repeated to form a plurality of ion implantation scan lines on the substrate till the scan area covers the whole substrate S. In FIG. 1a, the plurality of ion implantation scan lines is marked 1 and the arrow mark 100 represents the orientation of the substrate S.
Next, the substrate is rotated by 90 degree on the plan of the substrate S, and the ion beam is shifted with an interlace pitch T/2 (half scan pitch) along the second direction. And then the procedure of the ion implantation scan pass is repeated to form a plurality of ion implantation scan lines 2, which is perpendicular to the direction of the plurality of ion implantation scan lines 1, and the dotted lines represent the second scan path 22 as shown in FIG. 1b. 
The step shown in FIG. 1b is then further repeated two times, so as to complete the third time and the forth time of the ion implantation scan pass to form another plurality of ion implantation scan lines 1 and 2, as shown in FIG. 1c and FIG. 1d, respectively. Rotate the substrate S by 180 degree (i.e. continuous two rotations of 90 degree). The ion implantation scan lines formed by the first time of the ion implantation scan pass overlaps the ion implantation scan lines formed by the third time, and the ion implantation scan lines formed by the second time overlaps the ion implantation scan lines formed by the forth time. Therefore the third time and the forth time of the ion implantation scan pass are still marked 21 and 22, and the plurality of ion implantation scan lines formed are also still marked 1 and 2.
The dose of the ion implantation on the surface of the substrate S is illustrated in FIG. 2, where D(x) represents the distribution variation function of the dose. The larger value of D(x) is, and the worse the uniformity is. On contrary, the smaller value of D(x) is, the better the uniformity is.
For one ion implantation scan line, the farther the ion beam is away from the center of the ion implantation scan line, the smaller the dose of the ion implantation is. The first time of the ion implantation scan pass overlapping the third time of the ion implantation scan pass and the second time of the ion implantation scan pass overlapping the forth time of the ion implantation scan pass renders a larger value of the distribution variation function D(x), resulting in a poor uniformity.